feat: add PyPI package download statistics and popularity analysis tools

[loonghao]

🚀 New Features

This PR adds comprehensive PyPI package download statistics and popularity analysis tools to the MCP server.

✨ What's New

📊 Download Statistics Tools

• get_download_statistics - Get comprehensive download statistics for any package
• get_download_trends - Analyze download trends and time series data (last 180 days)
• get_top_downloaded_packages - Get the most popular packages by download count

🔧 Core Infrastructure

• PyPIStatsClient - New async client for pypistats.org API integration
• Advanced analysis functions - Download trends analysis with growth indicators
• Repository information integration - Links to GitHub/GitLab repositories
• Comprehensive caching - Efficient data caching for better performance

🎯 Use Cases

Now you can ask questions like:

• "What are the download statistics for the requests package this month?"
• "Show me the download trends for numpy over the last 180 days"
• "What are the top 10 most downloaded Python packages today?"
• "Compare the popularity of Django vs Flask vs FastAPI"
• "Which web framework has the highest download count this week?"

📈 Example Output

{
  "package": "requests",
  "downloads": {
    "last_day": 1500000,
    "last_week": 10500000,
    "last_month": 45000000
  },
  "analysis": {
    "total_downloads": 57000000,
    "highest_period": "last_month",
    "growth_indicators": {
      "daily_vs_weekly": 1.0,
      "weekly_vs_monthly": 0.93
    }
  },
  "metadata": {
    "name": "requests",
    "version": "2.31.0",
    "summary": "Python HTTP for Humans.",
    "project_urls": {
      "Repository": "https://github.com/psf/requests"
    }
  }
}

🧪 Testing

• ✅ Comprehensive unit tests with 76% coverage
• ✅ Mock-based testing for reliable CI/CD
• ✅ Integration tests for all new MCP tools
• ✅ Demo script with real-world examples

📚 Documentation

• ✅ Updated README with new features and examples
• ✅ Comprehensive docstrings for all functions
• ✅ Usage examples and conversation patterns
• ✅ Demo script (examples/download_stats_demo.py)

🔄 Backward Compatibility

• ✅ All existing functionality remains unchanged
• ✅ No breaking changes to existing APIs
• ✅ New features are additive only

📦 Files Changed

• pypi_query_mcp/core/stats_client.py - New PyPIStatsClient for API integration
• pypi_query_mcp/tools/download_stats.py - Download statistics tools implementation
• pypi_query_mcp/server.py - New MCP tools registration
• tests/test_download_stats.py - Comprehensive test coverage
• examples/download_stats_demo.py - Demo script with examples
• README.md - Updated documentation

🎉 Ready for Use

This feature is production-ready and can be used immediately after merging. The pypistats.org API is stable and widely used by the Python community.

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Note: This implementation uses the pypistats.org API which provides download statistics for the last 180 days. For longer historical data, users can be directed to use Google BigQuery with PyPI datasets.